Two stage extraction Time of Flight mass spectrometers are well known. The basic equations that describe two stage extraction Time of Flight mass spectrometers were first set out by Wiley and McLaren (W. C. Wiley and I. H. McLaren “Time-of-Flight Mass Spectrometer with Improved Resolution”, Review of Scientific Instruments 26, 1150 (1955)). The principles apply equally to continuous axial extraction Time of Flight mass analysers, orthogonal acceleration Time of Flight mass analysers and time lag focussing instruments.
FIG. 1 illustrates the principle of spatial (or space) focussing whereby ions 1 with an initial spatial distribution are present in an orthogonal acceleration extraction region located between a pusher electrode 2 and a first extraction grid electrode 3. The ions in the orthogonal acceleration extraction region are orthogonally accelerated through the first grid electrode 3 and then pass through a second grid electrode 4. The ions then pass through a field free or drift region and are brought to a focus at a plane 5 which corresponds with the plane at which an ion detector is positioned. The region between the pusher electrode 2 and the first grid electrode 3 forms a first stage extraction region and the region between the first grid electrode 3 and the second grid electrode 4 forms a second stage extraction region. The two stage extraction regions enable the instrumental resolution to be improved. The plane 5 of the ion detector is also known as the plane of second order spatial focus.
An ion beam with initial energy ΔVo and with no initial position deviation has a time of flight in the first acceleration stage (i.e. the first stage extraction region which is also referred to as the pusher region) given by:
                    t        =                              1            a                    ⁢                                                                      2                  ⁢                                                                          ⁢                  q                                m                                      ·                          [                                                                    (                                                                  V                        p                                            ±                                              Δ                        ⁢                                                                                                  ⁢                        Vo                                                              )                                                        1                    /                    2                                                  ±                                  Δ                  ⁢                                                                          ⁢                                      Vo                                          1                      /                      2                                                                                  ]                                                          (        1        )            wherein m is the mass of the ion, q is the charge, a is the acceleration and Vp is the potential applied to the pusher electrode 2 relative to the potential of the first grid electrode 3.
The initial velocity vo is related to the initial energy ΔVo by the relation:
                    vo        =                                                            2                ·                Δ                            ⁢                                                          ⁢              Vo                        m                                              (        2        )            
The second term in the square brackets of Eqn. 1 is referred to as the “turnaround time” which is a major limiting aberration in the design of Time of Flight mass analysers. The concept of turnaround time will now be discussed in more detail with reference to FIGS. 2A and 2B.
Ions that start at the same position within the orthogonal acceleration extraction region but which possess equal and opposite velocities will have identical energies in the flight tube given by:
                              K          ·          E                =                  qVacc          +                                    1              2                        ⁢                          mv              2                                                          (        3        )            
However, ions having equal and opposite initial velocities will be separated by the turnaround time Δt. The turnaround time is relatively long if a relatively shallow or low acceleration field is applied (see FIG. 2A). The turnaround time is relatively short if a relatively steep or high acceleration field is applied (see FIG. 2B). It is apparent from comparing FIG. 2B with FIG. 2A that Δt2<Δt1.
Turnaround time is often the major limiting aberration in designing a Time of Flight mass spectrometer and instrument designers go to great lengths to attempt to minimise this effect which results in a reduction in the overall resolution of the mass analyser.
A known approach to the problem of the aberration caused by the turnaround time is to accelerate the ions as forcefully as possible i.e. the acceleration term a in Eqn. 1 is made as large as possible by maximising the electric field. As a result the ratio Vp/Lp is maximised. Practically, this is achieved by making the pusher voltage Vp as high as possible and keeping the width Lp of the orthogonal acceleration extraction region as short as possible. In a known mass spectrometer the distance between the pusher electrode 2 and the first grid electrode 3 is <10 mm.
However, the known approach has a practical limit for a two stage extraction Time of Flight mass analyser since Wiley McLaren type spatial focussing necessitates that the Time of Flight mass analyser has a short field free region L3. As shown in FIG. 3, if the field free region L3 is relatively short then the flight times of ions through the field free region L3 will also be correspondingly short. This is highly problematic since it requires very fast, high bandwidth detection systems and hence it is impractical to increase the ratio Vp/Lp beyond a certain limit.
In order to improve the resolution of a Time of Flight mass analyser by adding a reflectron. The addition of a reflectron has the effect of re-imaging the first position of spatial focus at the ion detector as shown in FIG. 4 leading to longer practical flight time instruments which are capable of very high resolution. Reference is made to Dodonov et al., European Journal of Mass Spectrometry Volume 6, Issue 6, pages 481-490 (2000).
However, the addition of a reflectron to a Time of Flight mass spectrometer adds complexity and expense to the overall design of the instrument.
It is desired to provide a Time of Flight mass analyser which has a relatively high mass resolution but which does not necessarily include a reflectron.